1) Field of the Invention
The present invention relates generally to grain moisture sensors. More specifically, the present invention relates to an improved grain moisture sensor for combines.
2) Related Art
Grain moisture sensors have been used in combines, particularly in precision agriculture applications. Continuous or instantaneous grain moisture readings allow an operator to observe the moisture of the grain as it is being harvested. In conjunction with a GPS unit, a moisture sensor can be used to provide moisture mapping. In addition, moisture sensors are used in yield monitoring applications. When used in combination with a grain flow sensor, the moisture sensor information is often used to calculate the number of dry bushels in a field and the number of bushels per acre based on the number of wet bushels and the moisture content.
Moisture sensors in combines are commonly mounted in one of two places. The first of these places is in the grain tank auger. The grain tank auger is also known as the loading auger in a combine. There are a number of problems with mounting the moisture sensor in this location. The first is that in order to mount the moisture sensor the flighting of the loading auger must be removed. With removed flighting, material can build up which requires the operator to clean the sensor. If the moisture sensor is not kept clean,readings may be inaccurate or the moisture sensor may be inoperable.
A further problem with mounting the moisture sensor in the loading auger of a combine is the lag time or delay encountered when measuring moisture. When the moisture sensor is mounted in the loading auger position, moisture sensor readings are not taken until the grain is actually in the loading auger of the combine. Therefore, grain must travel up the elevator and fill the sump of the transition housing before the auger is able to deliver grain to the sensor and a moisture measurement can be taken. This deficiency frustrates the use of a moisture sensor in precision agriculture applications, making it more difficult to correctly associate a particular field location with a particular grain moisture.
A further problem with mounting grain moisture sensors in a loading auger is that such a moisture system does not provide for determining when there is sufficient grain present for a grain moisture measurement. Grain moisture sensors usually include capacitive plates. The volume between the plates must be covered before an accurate grain moisture measurement can be made. A moisture sensor that is not filled with grain is not accurately measuring the moisture of the grain. Therefore, this inability to know when the capacitive plate is covered can result in erroneous grain moisture measurements.
Another location that has been used to mount grain moisture sensors is on the side of the clean grain elevator. The clean grain elevator mounting location is thought to provide a steadier flow of grain. Further, the clean grain elevator location may avoid causing accelerated wear of the auger assembly and does not obstruct grain flow in the manner which the loading auger location may. Despite these improvements, a number of problems remain with mounting a moisture sensor on the side of the clean grain elevator in a combine. One problem relates to the slow cycle time of the moisture sensor. In a low flow condition which is not uncommon in grain harvesting, the sensor can be extremely slow to fill. For example, it may take up to four minutes to fill the sensor. Therefore, the number of moisture sensor readings is reduced and the moisture sensor data is insufficient for providing accurate measurements for moisture maps, yield determinations, and other purposes.
A further problem with mounting moisture sensors on the side of the clean grain elevator relates to the sensitivity of this mounting location in the presence of side slopes. It is not uncommon for a combine to be operating on a hill or slope. When the combine is operated on a slope such that the grain flow is directed away from the moisture sensor inlet, it is nearly impossible to fill the grain moisture sensor with sufficient grain to make a moisture determination.
A further problem with mounting moisture sensors on the clean grain elevator relates to grain leaks. When mounted on the side of the clean grain elevator, any grain leaks that occur result in the leaking grain spilling on the ground, as the grain leaks are not contained.
Another problem in grain moisture sensing relates to the sensor cell. Typically, the sensor cell consists of a parallel plate capacitor in which the grain serves as the dielectric material. The cell capacitance and therefore the permittivity of the grain between the plates is measured. From this measurement, the moisture of the grain is determined. Normally, these cell designs are not as close to an ideal parallel plate capacitor as desired. In particular, prior art designs for grain moisture sensors for use in combines use cells that are subject to electric field fringe effects. A fringe effect occurs when electric field lines are not both straight and perpendicular to the plates of the parallel plate capacitor. These fringe effects produce an uncontrollable influence on the measurements from material other than grain that is close to the cell but outside of the cell. Another problem with cell designs is that they do not produce uniformly dense electric field lines between the parallel plates. The nonuniform electric field density creates the problem of unequal sensitivity to grain throughout the cell. Thus the measurements of the moisture of the grain within the cell are not as accurate as desired in these respects.
Another problem relating to the prior art relates to the method for measuring cell capacitance. Measuring the capacitance of a cell filled with grain is a traditional way of obtaining grain moisture. There are two common prior art methods for measuring cell capacitance. The first method is to sense the changes in frequency of a variable oscillator that uses cell capacitance as one of its frequency determining elements. The second method is to excite the cell capacitance with a signal having a known frequency and to measure the absolute value of the resulting cell current, usually with a bridge type of circuit and a peak detector, and then to calculate the capacitance of the cell. Both of these methods tend to be dependent on grain cell construction and are sensitive to noise, changes in circuit characteristics, and parasitic effects. The first method also has the problem of poor control of frequency, especially as moisture varies. Both of these methods are also single dimensional, lacking the ability to measure both the dielectric and the loss properties of the grain. Therefore numerous problems remain with this type of sensing.
The combination of the dielectric and loss parameters is known as the complex permittivity. Complex permittivity is an intrinsic, frequency dependent material property. The knowledge of the grain""s complex permittivity at more than one frequency has been found to be a part of advanced moisture level assessment as has been demonstrated by USDA studies. Despite this observation, problems remain.
It is therefore an object of the present invention to provide a grain moisture sensor for use on a combine that improves upon the state of the art.
It is another object of the present invention to provide a grain moisture sensor that provides accurate and consistent grain moisture measurements.
It is a further object of the present invention to provide a grain moisture sensor that does not require the removal of flighting in the loading auger for cleaning.
Yet another object of the present invention is to provide a grain moisture sensor that avoids lags in time between when grain is harvested and when the moisture measurement is taken.
A further object of the present invention is to provide a grain moisture sensor that is capable of determining when the sensor cell is full.
Yet another object of the present invention is to provide an improved grain moisture sensor that is less affected by low flow conditions.
Yet another object of the present invention is to provide a grain moisture sensor for use in a combine that is insensitive to changes in the side slope of the ground being harvested.
Yet another object of the present invention is to provide a grain moisture sensor that contains grain leaks.
A still further object of the present invention is to provide a grain moisture sensor with a cell that has characteristics closer to an ideal parallel plate capacitor.
A still further object of the present invention is to provide a grain moisture sensor that provides for uniform electric field density to allow for equal sensitivity to grain throughout the cell.
Yet another object of the present invention is to provide a grain moisture sensor with a cell for reducing fringe effects produced by material other than grain that may be close to, but outside of the cell.
Yet another object of the present invention is to provide a grain moisture sensor that provides for increased protection from electromagnetic interference.
A still further object of the present invention is to provide a grain moisture sensor that provides for the measurement of complex permittivity of the grain.
Another object of the present invention is to provide a grain moisture sensor that provides for the measurement of complex permittivity of the grain at more than one frequency.
A grain moisture sensor of the present invention provides for the sensing of the moisture of grain being harvested by a combine. One aspect of the present invention relates to the location of the grain moisture sensor on the combine. According to the present invention, the grain moisture sensor is mounted off of the front of the clean grain elevator transition housing inside of the grain tank. This provides the advantages of access to the grain moisture sensor if required and the advantage that all leaks are contained. A further advantage is that the grain moisture sensor fills positively with grain. Further, this location of the grain moisture sensor allows for the sensor to always be filled regardless of the slope conditions of the combine.
Another aspect of the present invention relates to the cell design of the sensor. The cell of the present invention includes a driven plate to which excitation voltages are applied, a sense plate proximate and parallel to the driven plate for measuring current that passes through the cell, a fill plate adjacent to the sense plate for determining when the cell is full, and a guard adjacent to the sense plate and the fill plate for protecting the sense plate and the fill plate. The guard is electrically isolated from, but at the same potential as a sensed plate. The guard is parallel to and dimensionally larger than the sense plate in order to shape the electric field. The presence of the guard plate provides the advantage of straight electric field lines perpendicular to the sense plate and of uniform density throughout the region between the parallel plates. This results in reduced fringe effects and uniform electric field density allowing for equal sensitivity to grain throughout the cell. In addition, the guard shields the sense plate from external electric fields generated by sources other than the driven plate. The fill plate provides the advantage of accurate determination of whether or not the cell is full.
A further aspect of the present invention is the method in which the capacitance of a cell filled with grain is measured. The present invention provides for measurement of the complex permittivity of the grain. Further, the present invention provides for measurement of the complex permittivity at more than one frequency. This provides the advantage of permitting compensation for variations in grain density and conductivity effects which is particularly important in mobile moisture sensing applications such as the use of a moisture sensor on a combine. According to this aspect of the present invention, the circuit measures the real and imaginary components of both the excitation voltage and the sense current. From these values, the complex admittance of the cell is calculated. The measurements are repeated for the empty cell and the cell filled with grain. When the empty cell is not available, the calibrated reference admittances are used instead. The grain complex permittivity can then be calculated from these measurements. Mixers are used in the measurement of real and imaginary components of the voltage and current. This synchronous detection method has a very narrow band filtering effect, greatly reducing noise influence on the measurement. A virtual ground method of measuring low-level currents is used to provide the advantage of a substantial reduction in the influence of parasitic elements at the current sensing node. In addition, measurements can be corrected with the calibrated references to compensate for any environmental changes that may influence the circuit characteristics. This provides the advantage of securing stable and repetitive results.
In this matter, the present invention provides advantages in an improved grain moisture sensor.